Several different types of television broadcast formats and standards are used in different locales throughout the world. Two of the most pervasive are the system used in the United States of America known as NTSC (National Television Standards Committee) and the European system known as
(Phase Alternating Line). In addition, several systems for providing increased picture resolution or definition, generally referred to as HDTV (High Definition Television) have been and are continuing to be developed.
While the television receivers operating in these varied systems are equally varied, certain aspects remain generally similar. For example, most television receivers include circuitry for selecting the desired television signal from among a plurality of broadcast signals available, a signal processing system which recovers the picture and sound information from the broadcast signal, systems for sequentially scanning a display device such as a cathode ray tube in both horizontal and vertical directions, and scan synchronization systems operative upon the display to coordinate display scanning to the picture and sound information.
Despite significant differences between the signal selection and signal processing functions of television receivers operating in accordance with the above-mentioned variety of broadcast systems, the functions of display scanning and synchronization are generally quite similar. Generally, picture and sound information together with scan synchronizing signals are modulated upon a broadcast carrier at the transmitter. At the receiver, the scan synchronizing signals are separated from the remainder of the picture and sound information and used to control locally generated horizontal and vertical scan signals. The latter are used to drive the display scanning circuits of the display system.
Computer monitors and many video game devices are also similar to television receivers in that a display system, such as a cathode ray tube, is scanned in synchronism with picture information. In such systems, the scan signals are computer generated and are used to synchronize display scan and picture information in much the same manner as television receivers.
One of the most demanding aspects of display system synchronization is that of controlling the horizontal scan oscillator. This critical function is made challenging by the need for precise control of both the frequency and phase of the horizontal scan oscillator. One of the most common and pervasive horizontal oscillator control systems used in both television receivers, computer monitors, or video games is generally referred to as a phase locked loop. While a variety of different phase locked loop systems have been developed, the prior art system shown in FIG. 1 is typical and provides a helpful basis for understanding the basic problems associated with horizontal oscillator control.
With reference to FIG. 1, a typical phase locked loop system is constructed in accordance with the prior art and generally referenced by numeral 10. A sync signal separator 12 operates in accordance with conventional fabrication techniques to extract the horizontal scan synchronizing signals from the remainder of the received television signal to provide a reference signal input to a conventional phase detector 11. Phase detector 11 includes a pair of inputs 13A coupled to sync separator 12 and 13B coupled to the horizontal drive signal. Phase detector 11 produces an output signal at output 14 representative of the difference between inputs 13A and 13B. The output of phase detector 11 is coupled to a low pass filter 16 which in turn is coupled to an error amplifier 17. A voltage controlled oscillator 18 having an output frequency determined in part by the error signal applied by error amplifier 17 produces a periodic output signal which is coupled to a frequency divider 19. The output of frequency divider 19 is applied to input 13B of phase detector 11 and to the horizontal scan system.
In operation, phase detector 11 compares the output signal of frequency divider 19 to the reference synchronizing signals at input 13A. An error voltage indicative of the difference therebetween is filtered by low pass filter 16 and amplified by error amplifier 17 to produce a controlling voltage for voltage controlled oscillator 18. In the event voltage controlled oscillator 18 is precisely synchronized to the referenced synchronizing signals at input 13A, the output voltage of phase detector 11 is zero and the frequency of voltage controlled oscillator 18 remains unchanged. In practice, however, this condition seldom exists and, more typically, the frequency of voltage controlled oscillator 18 differs from that of the incoming reference signals. If the difference between oscillator 18 and the reference signals detected by phase detector 11 is a phase difference or minor frequency difference, the error voltage coupled to oscillator 18 by filter 16 and amplifier 17 causes oscillator 18 to change frequency in the direction which brings its output signal toward synchronization with the reference sync signals. It has been found that prior art systems of the type shown in FIG. 1 respond well to minor variations of phase and frequency between oscillator 18 and the reference synchronizing signals.
If, however, a substantial frequency difference exists between the frequency of signals produced by oscillator 18 and the reference synchronizing signals, a correspondingly large error voltage is produced by phase detector 11 which is amplified by error amplifier 17 to produce a substantial change of frequency in voltage control oscillator 18. As is well known, conventional phase locked loop systems respond to large frequency differences by reaching an equilibrium point in which a sufficient error voltage is maintained by the phase detector to provide the necessary control of the oscillator. This equilibrium results in a condition in which the frequency of oscillator 18 is sychronized to that of the incoming reference sync signals while a phase difference or phase error between oscillator output signals and sync signals remains. This phase error is referred to in the art as static phase error. In systems of the type shown in FIG. 1, the ability of the system to make large frequency compensations is accompanied by correspondingly large static phase errors. Thus, practitioners in the art generally must compromise system performance to provide the necessary frequency compensation characteristic at the expense of static phase error.
In attempting to minimize or overcome the need for such compromise of system performance, practitioners in the art have endeavored to provide improved systems. Such attempts have included multiple loop control systems and systems which alter the loop gain in response to frequency lock or out of lock conditions. While such attempts have improved certain aspects of the system performance, they have often been beset by difficulties associated with increased complexity or transition difficulties between the in-sync and out-of-sync condition of system operation. There remains, therefore, a need in the art for an improved horizontal scan oscillator control system which minimizes static phase error.
Accordingly, it is a general object of the present invention to provide an improved horizontal scan oscillator control system. It is a more particular object of the present invention to provide an improved oscillator control system which responds to and corrects static phase error.